Vue 2 failing to retain component data upon page refresh

Question

Vue 2 failing to retain component data upon page refresh

I've encountered a peculiar issue with Vue (v2.6.14) where I'm trying to create a new array based on one received as a prop. Here's the code snippet that's causing the trouble:

props: { employees: Array },
data() {
  return {
    sortedEmployees: [],
  };
},
mounted() {
  this.sortedEmployees = this.employees.slice(0);
},

The main goal here is to generate a separate reference for the employees array so that I can sort it for later display without modifying the original array. I don't need to clone the actual objects inside the array since I won't be changing them.

The issue arises when the app 'hot reloads' due to a code change – everything works perfectly, the hook is triggered and the component data is set correctly. However, when I refresh the page in the browser, even though the hook is still executed, the component data isn't properly set, resulting in an empty array. One way to resolve this is by setting up a watcher for the prop to ensure the data is properly updated, but my focus here is on understanding why this discrepancy occurs. If the hook is being called during a page refresh, why doesn't it set the data as expected, like it does during 'hot reloading'?

I'm working with a simple setup created using vue-cli without any complex configurations. Any insights into what might be causing this behavior would be greatly appreciated.

javascript vue.js

Answer 1

Answer №1

It seems like the employees are being loaded asynchronously, correct?

Without knowing the exact structure of your application, the issue typically arises as follows:

The mounted hook is triggered when the component mounts. If the employees are loaded asynchronously in the parent component, the mount hook is called before the async call is resolved, resulting in an empty array being copied at that time.

A watcher can resolve this issue by triggering once the async call is completed (updating the employees array).

A similar scenario occurs during hot reloads. When a hot reload happens, the mounted hook runs again – but this time the employees array is already filled with values, ensuring the correct array is copied in the mount hook.

Update

If you prefer to avoid using watchers, you can wrap your component like this:

<your-component
  v-if="employees.length > 0"
  :employees="employees"
/>

Keep in mind that the copied employees array within your component is still not reactive; it simply duplicates the array when it has more than one value. Using a watcher would be more advisable in this case.

If you opt for watchers, ensure to include the immediate: true option. This guarantees that the watcher is also triggered upon rendering (and hot reloads).

Summary

If you truly need to duplicate the array, utilize watchers (with the immediate: true flag).

For sorting purposes only, consider utilizing the computed property solution recommended by @Sebastian Scholl.

Answer 2

It seems like the employees are being loaded asynchronously, correct?

Without knowing the exact structure of your application, the issue typically arises as follows:

The mounted hook is triggered when the component mounts. If the employees are loaded asynchronously in the parent component, the mount hook is called before the async call is resolved, resulting in an empty array being copied at that time.

A watcher can resolve this issue by triggering once the async call is completed (updating the employees array).

A similar scenario occurs during hot reloads. When a hot reload happens, the mounted hook runs again – but this time the employees array is already filled with values, ensuring the correct array is copied in the mount hook.

Update

If you prefer to avoid using watchers, you can wrap your component like this:

<your-component
  v-if="employees.length > 0"
  :employees="employees"
/>

Keep in mind that the copied employees array within your component is still not reactive; it simply duplicates the array when it has more than one value. Using a watcher would be more advisable in this case.

If you opt for watchers, ensure to include the immediate: true option. This guarantees that the watcher is also triggered upon rendering (and hot reloads).

Summary

If you truly need to duplicate the array, utilize watchers (with the immediate: true flag).

For sorting purposes only, consider utilizing the computed property solution recommended by @Sebastian Scholl.

Answer 3

Answer №2

It seems like the component is refreshing (resetting) when the prop changes, but it is not remounting. This is why the array resets to its default state ([]), whereas on hot-reload the entire page reloads.

Try the following, and if it doesn't resolve the issue, I recommend using a Watcher.

<template>
  <div>
    sorted: {{ sortedEmployees }}
    <br />
    not sorted: {{ employees }}
  </div>
</template>

<script>
export default {
  props: {
    employees: Array,
  },
  data() {
    return {
      sortedEmployees: Array.from(this.employees),
    };
  }
};
</script>

Another approach is to create a Computed method and add any filtering/sorting logic inside that method:

<template>
  <div>
    <input v-model="sortKey" />
    sorted: {{ sortedEmployees }}
  </div>
</template>

<script>
export default {
  props: {
    employees: Array,
  },
  data () {
    return {
      sortKey: ''
    }
  },
  computed: {
    sortedEmployees() {
      return Array.from(this.employees).sort(this.sortingFunction);
    },
  },
  methods: {
    sortingFunction(a, b) {
      // sorting function using this.sortKey
    }
  }
};
</script>

UPDATED ANSWER

I made some changes to the example code and managed to get it working as you described.

App.js I updated the code so that the employees array is updated after 3 seconds:

<template>
  <div id="app">
    <Dashboard :employees="employees" />
  </div>
</template>

<script>
import Dashboard from './components/Dashboard.vue';

export default {
  name: 'App', 

  components: { Dashboard },

  data () {
    return {
      employees: [
        {
          employeeId: '1',
          firstName: 'Leite',
        }
      ]
    };
  },

  methods: {
    updateEmployees () {
      this.employees = this.employees.concat([
        {
          employeeId: '2',
          firstName: 'Jacinto',
          
        },
        {
          employeeId: '3',
          firstName: 'Capelo',
        }
      ]);
    }
  },
  
  mounted () {
    setTimeout(this.updateEmployees, 3000)
  },
};
</script>

Dashboard.js The updated() lifecycle hook runs whenever data changes are detected (props and data properties). This effectively detects the change in the prop passed by the parent App.js component and re-renders the data - unlike the mounted hook which only runs once per page load.

<template>
  <div id="dashboard">
    <div
      v-for="(employee, index) in sortedEmployees"
      :key="employee.employeeId"
    >
      {{ employee.firstName }}
    </div>
  </div>
</template>

<script>

export default {
  name: 'Dashboard',
  props: { 
    employees: Array 
  },
  data() {
    return {
      sortedEmployees: Array.from(this.employees)
    };
  },
  updated() {
    this.sortedEmployees = Array.from(this.employees)
  }
};
</script>

Answer 4

It seems like the component is refreshing (resetting) when the prop changes, but it is not remounting. This is why the array resets to its default state ([]), whereas on hot-reload the entire page reloads.

Try the following, and if it doesn't resolve the issue, I recommend using a Watcher.

<template>
  <div>
    sorted: {{ sortedEmployees }}
    <br />
    not sorted: {{ employees }}
  </div>
</template>

<script>
export default {
  props: {
    employees: Array,
  },
  data() {
    return {
      sortedEmployees: Array.from(this.employees),
    };
  }
};
</script>

Another approach is to create a Computed method and add any filtering/sorting logic inside that method:

<template>
  <div>
    <input v-model="sortKey" />
    sorted: {{ sortedEmployees }}
  </div>
</template>

<script>
export default {
  props: {
    employees: Array,
  },
  data () {
    return {
      sortKey: ''
    }
  },
  computed: {
    sortedEmployees() {
      return Array.from(this.employees).sort(this.sortingFunction);
    },
  },
  methods: {
    sortingFunction(a, b) {
      // sorting function using this.sortKey
    }
  }
};
</script>

UPDATED ANSWER

I made some changes to the example code and managed to get it working as you described.

App.js I updated the code so that the employees array is updated after 3 seconds:

<template>
  <div id="app">
    <Dashboard :employees="employees" />
  </div>
</template>

<script>
import Dashboard from './components/Dashboard.vue';

export default {
  name: 'App', 

  components: { Dashboard },

  data () {
    return {
      employees: [
        {
          employeeId: '1',
          firstName: 'Leite',
        }
      ]
    };
  },

  methods: {
    updateEmployees () {
      this.employees = this.employees.concat([
        {
          employeeId: '2',
          firstName: 'Jacinto',
          
        },
        {
          employeeId: '3',
          firstName: 'Capelo',
        }
      ]);
    }
  },
  
  mounted () {
    setTimeout(this.updateEmployees, 3000)
  },
};
</script>

Dashboard.js The updated() lifecycle hook runs whenever data changes are detected (props and data properties). This effectively detects the change in the prop passed by the parent App.js component and re-renders the data - unlike the mounted hook which only runs once per page load.

<template>
  <div id="dashboard">
    <div
      v-for="(employee, index) in sortedEmployees"
      :key="employee.employeeId"
    >
      {{ employee.firstName }}
    </div>
  </div>
</template>

<script>

export default {
  name: 'Dashboard',
  props: { 
    employees: Array 
  },
  data() {
    return {
      sortedEmployees: Array.from(this.employees)
    };
  },
  updated() {
    this.sortedEmployees = Array.from(this.employees)
  }
};
</script>

Vue 2 failing to retain component data upon page refresh

Answer №1

Answer №2

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